Lawn watering is generally anything but a precise process. Many people water when the lawn looks dry or when they think of it. Alternatively, a watering system may be on a timer, whereby the lawn is watered at timed intervals, irrespective of whether or not it needs watering.
Apparatus to determine moisture content in various substances have been available in different forms and with varying degrees of success. For example, there are some devices which measure electrical resistance in the soil but these have generally not been satisfactory because resistance can vary with the type of elements in the soil and with the density of material. Thus the resistance value is not necessarily a true indication of percentage moisture. Further, the corrosive effects of soil may cause deleterious results on the electrodes used in such gauges. Tensiometers have been used, comprising a sealed, water-filled tube with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. These are best suited to sandy soils but give sporadic readings if the soil becomes too dry. Tensiometers also tend to require frequent servicing.
Electrical resistance blocks or gypsum blocks are made with gypsum formed around a pair of stainless steel wires or wire grids that may be connected to a resistance meter. The moisture content of the gypsum, when placed in soil, approximates the moisture content in the soil and the flow of electricity between the wires indicates soil moisture content. Gypsum blocks are not particularly useful in sandy soils.
Neutron probes are relatively expensive but are more accurate than most other moisture sensors. This device employs a radioactive source emitting neutrons that are slowed by collisions with hydrogen atoms, thereby indicating the relative water present. The procedure for using neutron probes is time consuming and requires special training in handling and using the equipment.
Finally, moisture content may be determined by gravimetric sampling, a time consuming process involving driving off the moisture in a soil sample by drying in an oven.
The least precise and likely most used test is feel and visual observation; an experienced irrragator can provide an estimate of moisture level with a reasonable and useful degree of accuracy.
A simple, inexpensive and practical system for accurately sensing moisture percentage in media such as earth, in order to provide indications which may be adapted for controlling a lawn watering system, has not previously been available.